In a wireless communication system, a sending end and a receiving end usually use multiple antennas to send and receive, so as to get a higher rate. A principle of multi-antenna technology is using some characteristics of a channel to form multi-layer transmissions matching the characteristics of the channel. Through the multi-antenna technology, a radiation direction of a signal is highly targeted, which can improve system performance effectively. A significant performance improvement is achieved without increasing bandwidth and power. Therefore, the multi-antenna technology is a very promising technology and widely applied in current systems. Data transmission performance of a multi-antenna system mainly depends on measurement and feedback of channel information, so the measurement and feedback of channel information is the core of the multi-antenna technology. How to ensure accuracy, overhead and robustness of channel measurement and channel information feedback becomes an important problem.
Basic contents related to Channel State Information (CSI) in a Long Term Evolution (LTE) system are as follows.
The basic contents include the content of CSI feedback. Implicit CSI feedback generally includes Channel Quality Indication (CQI), Precoding Matrix Indicator (PMI) and Rank Indicator (RI). The CQI is an indicator for measuring the quality of a downlink channel. In a protocol 36-213, the CQI is represented by integer values from 0 to 15, respectively representing different CQI levels; and different CQIs correspond to their respective Modulation and Coding Schemes (MCS) and encoding rates. The RI is used for describing the number of independent spatial channels, corresponding the rank of a channel response matrix. In an open-loop spatial multiplexing mode and a closed-loop spatial multiplexing mode, a piece of User Equipment (UE) is required to feedback RI information. In other modes, there is no need to feed back the RI information. The rank of the channel response matrix corresponds to the number of layers. The PMI feeds back the best precoding information, which indicates, based on an index feedback, a code word best matching the characteristic of the current channel in an agreed codebook.
The basic contents also include two modes of CSI feedback. There are mainly two modes of CSI feedback of a terminal. A base station may configure the terminal to measure and quantize the channel information, and periodically feedback the quantized CSI (including the RI, the PMI, and the CQI) through a Physical Uplink Control Channel (PUCCH). The base station may also trigger, if needed, the terminal to report the CSI (including the RI, the PMI, and the CQI) aperiodically and unexpectedly, so as to overcome the problem that real-time performance of periodical feedback is modest and CSI quantization accuracy is limited to the control of channel overhead.
The basic contents also include a CSI process. The 3GPP introduces a concept of CSI process. The base station may configure multiple CSI processes for the terminal. Each CSI process is equivalent to a CSI measurement and feedback process. Each CSI process is independent from other CSI processes, and parameters are configured separately. One process is supported in a transmission mode 9, and at most four processes may be supported in a transmission mode 10. The configuration of channel measurement part and the configuration of interference measurement part and feedback mode are defined in the configuration of each CSI process.
The basic contents also include a feedback class. The design of the measurement and feedback of the CSI is comparatively simple in an early LTE system version. Early CSI feedback only supports the configuration of a small set of pilot ports (2, 4, 8) and low-dimensional feedback, wherein all the pilot ports are non-precoding pilot. However, with the number of antennas is increasing and an accuracy requirement gets higher and higher, the deployment of the base station becomes more and more diverse, so a pilot overhead, a feedback overhead and complexity of feedback quantization increase significantly. A new definition of CSI feedback class is introduced in a new LTE version. There are two specific feedback classes, which are respectively class A and class B. The base station may configure the feedback class for each CSI process. The class A supports a high-dimensional 2D antenna topology (supporting more than eight ports), and the feedback is based on high-dimensional non-precoding pilot measurement, and uses the high-accuracy codebook. The class B supports the configuration of multiple precoding CSI Reference Signals (CSI-RSs) to perform beam selection (the number of pilots is greater than 1, and the total number of ports may be greater than 8), in such case, the CSI-RS sent by the base station is generally a precoding pilot; the UE may need to first perform precoding pilot selection or resource selection of the precoding pilot, and then perform, based on a selected CSI-RS resource, quantization feedback of the channel information, including resource selection information CSI-RS Resource Index (CRI) and the RI, PMI and CQI corresponding to the selected CSI-RS resource.
For some new feedback classes introduced, a pilot dimension corresponding to the feedback of class A is usually high, and more than eight ports, so complexity of channel estimation will increase exponentially; at the same time, a codebook dimension used in the process of channel quantization is large, the number of code words increases by several times compared with the previous number code words, so complexity of code word selection will be very high. The class B needs to perform channel measurement and interference measurement to multiple pilots, and needs to perform performance comparison after quantization to the channel information of multiple pilots, so complexity is also very high. Therefore, both the feedback of class A and the feedback of class B bring a big challenge to implementation complexity and cost of the terminal.